Field of Invention
The invention relates to waterproofed insulation materials and to a vaporization method for waterproofing porous thermal insulation to minimize weight penalties due to the effects of water imbibition. More particularly, the invention relates to waterproofing porous ceramic insulation by vapor deposition of silicone precursors onto the surface of the ceramic insulation. The precursors are vaporized on the insulation forming a protective silicone outer layer. The primary application of this invention is for waterproofing ceramic materials used in heat shields for space vehicles subjected to very high aeroconvective heating environments. The silicone coatings are particularly useful for ceramic materials such as those made of silicon carbide, aluminum oxide, zirconium oxide, aluminoborosilicate silicone dioxide and the like.
More specifically, this invention relates to silicone coatings for waterproofing insulation materials such as ceramic fabrics. This invention particularly relates to a reusable thermal structure comprising ceramic materials, as a substrate, having coated thereon the protective films of this invention which provides waterproofed films and resistance to high temperatures and thermal shock. The protective coatings may be applied on flexible ceramic blankets. Examples of these blankets include various rigid and flexible, porous thermal insulating materials which have been used for the thermal protection system on the space shuttle. These porous insulating materials included AFRSI, TABI, FRCI and AETB. The AFRSI (Advanced Flexible Reusable Surface Insulation) is a flexible composite blanket type of insulation having top or outer surface fabric layer of silica fibers and a bottom surface fabric layer of borosilicate glass, with a batting in the middle of 100% silica fibers. The TABI (Tailorable Advanced Blanket Insulation) is also a flexible composite having a top and bottom surface fabric layer of silicon carbide fiber, with an interior fill or batting of Saffil (5% SiO.sub.2 and 95% Al.sub.2 O.sub.3) fiber. FRCI (Fibrous Refractory Composite Insulation) is a rigid tile composed of a rigidized mixture of 78% silica and 22% aluminoborosilicate fibers. The AETB (Advanced Enhanced Thermal Barrier) is also a rigid tile type of thermal insulation and has a composition of 68% silica, 12% aluminoborosilicate and 20% alumina fibers. A more detailed discussion of these flexible blankets can be found in U.S. Pat. Nos. 5,038,693 and 5,277,959 issued to D. A. Kourtides et al., the disclosures of which are hereby incorporated in this application by reference. Thus, the primary application of this invention is to protect and waterproof ceramic materials e.g. ceramic blankets used in heat shields for space vehicles subjected to very high aeroconvective heating environments.
The ceramic materials, and particularly materials which contain one or more inorganic oxides such as silicates, aluminates, aluminosilicates, borates, phosphates, titanates and the like, have hygroscopic surfaces which adsorb or imbibe moisture. Moisture adsorption can be a problem if it adds significantly to the weight of the ceramic and if the ceramic is exposed to a high temperature environment. Thermal protection systems employ both flexible and rigid ceramic insulation, with the flexible insulation often comprising various layers fabricated of ceramic fibers which may or may not include layers of metal foil. These thermal protections systems are used on reentry space vehicles, such as the space shuttle which must leave and reenter the earth's atmosphere. The space shuttle requires light weight and vary thermally efficient, rigid and flexible exterior thermal protection systems which have to withstand a wide variety of environments, including temperatures, for example, of from 1000-1600.degree. C.
Specifically, space vehicles, such as the space shuttle which must leave and reenter the earth's atmosphere, require exterior thermal protection. The successful operation of the space shuttle requires the development of light weight and thermally efficient exterior thermal protection systems which have to withstand a wide variety of environments. These thermal protection systems (TPS) are used in the form of rigid surface insulation at high temperatures. Flexible blanket insulations are used at moderate high temperature and the oxidation protected, reinforced, rigid carbon/carbon materials are used at severe temperatures (up to -1600.degree. C.). During reentry into the earth's atmosphere, the TPS must maintain the vehicle's exterior structure below 175.degree. C. while experiencing substantial aeroconvective thermal environments which heats the surface of the TPS to these high temperatures. In space, the thermal protection must insulate the vehicle from the deep and constant cold (e.g., -70.degree. C.) experienced while in orbit. In addition to thermal and aeroconvective environments, the TPS must be able to withstand the mechanical stress associated with launch vibrations, acoustics structural movement of the surface of the vehicle and of the TPS material, and the landing impact. Except for the rigid, oxidation protected carbon--carbon composites used on the nose and other leading edges of a reentry space vehicle, typical TPS insulation is a rigid or flexible, porous ceramic or ceramic composite which can imbibe water comprising one or more thermally resistant oxides, carbides, borides, silicides, borosilicates and nitrides as disclosed, for example, in U.S. Pat. Nos. 5,038,693; 5,227,959 and 5,296,288, the disclosures of which are incorporated herein by reference.
The ceramic insulation materials currently in use include high purity silicon dioxide, aluminum oxide, silicon carbide, aluminosilicate, aluminoborosilicate and zirconium diboride. These ceramics are very porous and often have a void volume of over 90%. This degree of porosity creates problems. One problem relates to hot gas, that penetration into the ceramic in the high temperature aeroconvective environment encountered on reentry. If this occurs, it can melt the substrate and outer skin of the vehicle under the ceramic insulation. These materials also wet and can result in the porous insulation absorbing more than three to five times its own weight of water. Therefore, the insulation must be waterproofed so that it is unaffected when in contact with water in any form, including high humidity. In addition to adding unacceptable amount to the weight of the insulation, the presence of water in the ceramic insulation can create other problems, such as freeze-thaw damage and explosive vaporization on reentry into the earth's atmosphere. On one occasion, as a result of a hail storm many tiles on a Space Shuttle lost their waterproofing and picked up moisture, and therefore, the orbital time line of the vehicle had to be changed to provide a favorable Sun attitude to drive the water out of the tiles before ice damage could occur. Flexible ceramic insulation is more forgiving with respect to freeze/thaw damage, but the excess weight of the absorbed water is still a significant problem. Also, the insulation must be protected from contact with salt spray if it contains silica, as salt will devitrify silica at high temperatures and destroy the insulation. Another problem relates to embrittlement of the insulation in the high temperature aeroconvective environment encountered during reentry, which makes it very susceptible to damage. Attempts to resolve these problems were found to be costly, time consuming, and difficult to apply.
There are a number of other disadvantages in the methods used in the prior art. For example, the amount of waterproof film deposited must be on the order of tenths of a percent by weight of the insulation to prevent a weight penalty. Therefore, the addition of nonvolatile, relatively dense liquid reactants to the ceramic insulation and subsequent reaction can result in a substantial weight increase in the insulation. Methods of using a dilute system in which the reactants are added to a solvent carrier to lower the weight deposited has added complications. The carrier may contaminate the work area and environment and must be removed, disposed of, or recycled. In addition, the process of impregnation usually produces a nonhomogeneous film. Moreover, removing the carrier is an added operation and thereby increases the cost and may cause delays and result in an unacceptable weight penalty for the insulation.
Further, vapor phase hydrosilation also has been used to combine a silane functionality (--SiH) with another molecule containing vinyl functionality (--CH.dbd.CH.sub.2). This reaction can be facilitated by heat, peroxides, complexed metallic catalysts, U.V. light, and gamma radiation. By using the multi-functional silanes with the alkenes or alkynes, polymerization can occur on the insulation surface to form the silicone coatings. In addition, the silanes can react with a solid containing silanol groups to anchor the molecules to the surface. Alkoxysilanes are known to be used extensively for waterproofing ceramic insulation. These alkoxysilanes, however, are moisture sensitive and have some toxicity problems. The silane/vinyl systems are less reactive and considered more benign than the alkoxysilanes. The use of vapor or the CVD (chemical vapor deposition) technique of this invention assures better control over the quantity and location of film being deposited. Films can be made thin and are homogeneous by the CVD process. By this process films which are water repellent can be deposited on the insulation that are less than a percent by weight of the original insulation. Where a catalyst is used in the CVD system, the catalyst is added to, in contact with or in the proximity of the insulation to be waterproofed, but this catalyst need not touch or contaminate the insulation to be waterproofed.